This invention relates to fiber optic sensors and, more particularly, to marine seismic streamers using optical fibers for sensing changes in acoustic fields.
Marine seismic sensing devices are known that utilize discrete optical hydrophones which are assembled into marine seismic streamers. The discrete hydrophones use a pulsed laser to provide light to hydrophones made of optical fiber wound around mandrels. Pressure changes about a hydrophone cause deformations, which in turn cause phase modulation of light traveling through the fibers within each discrete hydrophone. Those changes are recorded as interference patterns produced at each discrete sensor. The individual interference patterns are coupled to a return cable to return to the shipboard for processing. Discrete optical hydrophones require a significant amount of fabrication, because each hydrophone must be spliced to optical coupler and return fibers, and the whole assembly encased and inserted into a hydrophone streamer skeleton. Marine seismic streamers of such individual sensors are bulky and expensive to fabricate.
Alternate types of optical hydrophone streamer systems are also known that utilize a streamer with discrete optical hydrophone sensors that operate by phase and intensity modulation of laser light input. Each sensor includes a mandrel-wound section of fiber coupled to two tails of optical fiber, each tail ending in an internal mirror. In this approach, light is reflected back and forth between the tails to produce phase and intensity modulation of the optical signal in response to sensed local acoustic pressure change. These two-tail systems have not been considered practical or economical for use in marine seismic streamers.
Such conventional optical sensing systems are also limited in their application by cross talk effects. For example, if the width of the pulse is less than the round-trip optical propagation delay in each sensor element, the output obtained through the optical coupler consists of a series of N+1 pulses that are separated in the time domain. Apart from cross-talk effects, these pulses contain no direct interferometric information. Application of this pulse train to a compensating interferometer of optical imbalance 2L coherently mixes pulses obtained from consecutive reflectors, thus generating the interferometric outputs from each sensor element. Cross talk then occurs between optical sensors due to multiple reflection paths. The cross talk manifests itself as side-bands in a heterodyne modulation and demodulation. In seismic acquisition, cross-talk of acoustic signals between sensors is highly deleterious to processing data. It is generally accepted that these crossfeed products must be kept below xe2x88x9290 dB in order to provide quality seismic data. To achieve this level of crossfeed the reflectivity of the mirrors would have to be so low that there would be inadequate returned optical power to process.
The present invention is directed to providing seismic optical sensor systems that overcome the limitations of existing systems.
According to one embodiment of the present invention, an optical sensor system is provided that includes an optical source for controllably generating pulses of light having a predetermined wavelength distribution, an optical coupler coupled to the optical source, an array of optical sensors coupled to the optical coupler, and a compensating interferometer coupled to the optical coupler for generating interference patterns representative of environmental conditions within the array of optical sensors.
According to another embodiment of the present invention, an optical source for use in an optical sensor system is provided that includes a laser, a modulator coupled to the laser, and a polarization controller coupled to the modulator.
According to another embodiment of the present invention, a polarization controller for use in an optical source is provided that includes a first optical conductor, a bias voltage source, a polarization scrambler coupled to the bias voltage source, a ramp voltage source coupled to the polarization scrambler, and a second optical conductor coupled to the polarization controller. The polarization of the first and second optical conductors are substantially equal.
According to another embodiment of the present invention, an array of optical sensors for use in an optical sensor system is provided that includes a sensing optical conductor and a plurality of partially reflective assemblies coupled to the sensing optical conductor. Wherein the partially reflective assemblies are equally distributed along the length of the sensing optical conductor.
According to another embodiment of the present invention, a compensating interferometer assembly for processing reflected pulses of light is provided that includes a first optical coupler, a first optical path coupled to the first optical coupler, a second optical path coupled to the first optical coupler and a second optical coupler coupled to the first and second optical paths. The optical travel time of the first optical path is greater than the optical travel time of the second optical path.
According to another embodiment of the present invention, a method of optically sensing environmental conditions is provided that includes generating pulses of light, reflecting the pulses of light at partially reflective locations, delaying the reflected pulses of light, and interfering the reflected pulses of light with the delayed reflected pulses of light to generate an interference pattern.
According to another embodiment of the present invention, a method of generating pulses of light is provided that includes generating light waves and blocking the light waves.
According to another embodiment of the present invention, a pulsed laser is provided that includes a laser having an output port and an optical switch operably coupled to the output port of the laser.
According to another embodiment of the present invention, an optical switch is provided that includes a first single polarization fiber, a polarization scrambler having an input port and an output port, wherein the input port of the polarization scrambler is operably coupled to the first single polarization fiber, and a second single polarization fiber operably coupled to the output port of the polarization scrambler.
The present embodiments of the invention provide optical sensing systems having enhanced operational performance through the use of features such as, for example, time division multiplexing and wave division multiplexing, optical amplifiers, and optical switches.